BIG PHYSICS, BIG QUESTIONS –

Science: Female infidelity – may the best sperm win

Why are so many females unfaithful to their partners? Two-timing would
not seem to have the advantages for them that it has males. A pair of British
zoologists think they know why it has evolved.

Mark Bellis and Robin Baker of the University of Manchester say females
mate with several males because this allows them to pit the sperm of different
males against each other in their reproductive tracts. In this way, they
ensure that they are fertilised by the best-quality sperm.

Male infidelity has always been easy to explain. Fertilising a female
takes relatively little time and energy, so a male can father a lot of children
quickly and easily. The more females a male mates with, the more descendants
he will have. His genes – including any that promote such ‘promiscuity’
– will become widespread.

Females, on the other hand, must devote a lot of time and energy to
each of their young, from producing the original egg, through nurturing
the fetus, to looking after the offspring until it becomes independent.
This means that the number of young a female can produce is limited. She
does not need to mate very often in order to reproduce at her maximum possible
rate.

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More important to a female than the number of times she copulates is
the quality of the males. She will increase the number of her descendants
if she chooses a male who can increase the success of her offspring – by
helping to rear them, or by passing on to them genes that make them big,
strong or attractive. Females are, therefore, choosy when it comes to selecting
a mate.

But if quality rather than quantity of partners is important to a female,
why does she not seek out the best male she can, and stick with him? Some
biologists have claimed that a female is unfaithful so that she can persuade
a second male to help rear her offspring. Others have claimed that if a
female bears children by a range of fathers, she increases their genetic
variability, maximising the chance that at least some will succeed.

However, Bellis and Baker have another idea for the evolution of female
infidelity, in the human species at least. In a world in which no individual
is strictly monogamous, they say, there is one particular characteristic
that is very advantageous to pass on to one’s sons: good-quality sperm capable
of beating any opposition.

But sperm quality if not something a female is likely to be able to
assess from the physical or behavioural attributes of a potential mate.
there is only one way to test the competitive ability of a male’s sperm,
say Bellis and Baker. She must mate with other males so that the sperm of
more than one has a chance of fertilising her egg.

This does not, of course, imply conscious scheming on the part of the
female, say the biologists. Any female who mates with several males will
automatically be fertilised by the most able sperm. If this trait is heritable,
then she will have more successful sons than other femals, and the behaviour
will spread throughout the species.

To test their theory, Bellis and Baker compiled a questionnaire to quiz
women on their infidelity and had it published in the magazine Company.
the researchers received replies from more than 3,000 women. Out of these,
162 had last copulated with someone other than their regular partner.

If the reason a female mates outside the normal pair bond is to gain
extra help with the young, it is important that the second male believes
that he has a genetic stake in the offspring. But, say Bellis and Baker,
whether or not he fertilises her is not important to the female. This means
that she would copulate with other males even during infertile stages of
her menstrual cycle.

When they studied the results of their questionnaire, Bellis and Baker
found that this was not the case. They found that women were most unfaithful
during their most fertile period, just before ovulation.

According to the biologists, this suggests that the evolutionary advantage
of this behaviour depends on the female conceiving offspring by the second
male. If could be that the ‘genetic variability’ idea is the correct one,
or that he looks as though he has particularly ‘good genes’.

Bellis and Baker looked at the copulations that occurred within five
days of the last mating with a regular partner. Significantly, this type
of copulation was also more closely associated with peak fertility than
any other.

Human sperm can last for at least five days in the female reproductive
tract. So if the female is to be fertilised by the most able sperm, she
can only pit them against each other if she mates with her second male within
five days of mating her regular partner. According to the biologists, females
act in a way that encourages competition between the sperm of different
males.

Bellis and Baker conclude that in the face of natural selection, infidelity
is an advantageous to females as it is to males, and that females time their
sexual behaviour in order to maximise the competitive abilities inherited
by their offspring. It may take two to tango – but in evolutionary terms
it is the females who call the tune (Animal Behaviour, vol 40, p 997).